Note:
PDF Password Remover for Mac can decrypt PDF files with Owner Password directly without knowing the password. But if the PDF file is encrypted with User/Open Password, you need to unlock it before decryption. Otherwise descryption of the file will be skipped.

Actually even Acrobat also requires the password if you want to remove pdf password on mac from encrypted PDF files. And most PDF decryption tools on the market can't remove Open Password from PDF files even if you know the password. It's reasonable because the Open Password is set by authors to protect copyrights of their works. You can't get access to the file without permission.

If you have permission to get access to the file and want to remove the pdf password for easier reading, just click on the lock icon beside the PDF file and type correct password in the pop-up box to unlock the PDF file for decryption.